enters a host cell and directs the production of the viral components, proteins and nucleic acids, needed to form new
virus particles calledvirions. New virions are made in the host cell by assembly of viral components. The new virions
transport the viral genome to another host cell to carry out another round of infection.Table 6.1summarizes the
properties of viruses.
Characteristics of Viruses
Infectious, acellular pathogens
Obligate intracellular parasites with host and cell-type specificity
DNA or RNA genome (never both)
Genome is surrounded by a protein capsid and, in some cases, a phospholipid membrane studded with viral
glycoproteins
Lack genes for many products needed for successful reproduction, requiring exploitation of host-cell genomes to
reproduceTable 6.1
Figure 6.2 (a) Tobacco mosaic virus (TMV) viewed with transmission electron microscope. (b) Plants infected with
tobacco mosaic disease (TMD), caused by TMV. (credit a: modification of work by USDA Agricultural Research
Service—scale-bar data from Matt Russell; credit b: modification of work by USDA Forest Service, Department of
Plant Pathology Archive North Carolina State University)
- Why was the first virus investigated mistaken for a toxin?
Hosts and Viral Transmission
Viruses can infect every type of host cell, including those of plants, animals, fungi, protists, bacteria, and archaea.
Most viruses will only be able to infect the cells of one or a few species of organism. This is called thehost range.
However, having a wide host range is not common and viruses will typically only infect specific hosts and only
specific cell types within those hosts. The viruses that infect bacteria are calledbacteriophages, or simply phages.
The wordphagecomes from the Greek word for devour. Other viruses are just identified by their host group, such
as animal or plant viruses. Once a cell is infected, the effects of the virus can vary depending on the type of virus.
Chapter 6 | Acellular Pathogens 231